Her husband Richard Ratcliffe said he had met Foreign Secretary Dominic Raab a couple of weeks ago and warned him there had been an escalation in foreign nationals being detained in Iran to be used as diplomatic bargaining chips.

"More people are being taken and the government needs to do a better job at protecting ordinary people from being held and used as chess pieces in this way," he told Sky News.

In a message to the families of Ms King and Mr Firkin, he said: "My heart goes out to you - it's a terrible situation to be involved in."

Sky news understands that another woman, reportedly a lecturer at an Australian university, was arrested last year. Her citizenship and situation are unclear.

The arrests come amid a souring in relations between Britain and Iran, after issues such as the seizure by the Royal Marines in July of an Iranian oil tankernear Gibraltar.

Two weeks later, the British-flagged oil tanker Stena Impero was seized by Iran for alleged marine violations in the Strait of Hormuz. Seven of the Swedish-owned vessel's 23-strong crew members have since been released.

A spokesperson for the Foreign and Commonwealth Office said on Wednesday: "The foreign secretary met the Iranian ambassador and raised serious concerns about the number of dual national citizens detained by Iran and their conditions of detention."

The FCO has been warning dual Iranian citizens not to travel to Iran, but news of the arrests is likely to raise questions about the level of danger to travellers of other nationalities too.

Its travel advice for Iran currently says: "There is a risk that British nationals, and a higher risk that British-Iranian dual nationals, could be arbitrarily detained in Iran.

"All British nationals should consider carefully the risks of travelling to Iran."

A spokesperson for Australia's Department of Foreign Affairs said: "The Department of Foreign Affairs and Trade is providing consular assistance to the families of three Australians detained in Iran.